Quote Originally Posted by drcampbell View Post
It's certainly better than not derating at all, but I wouldn't say it's "conservative", just adequate. At 3000 m altitude, there are 26% fewer air molecules in a given volume of air than at sea level.
While it's true that the air is thinner (less dense) than compared to sea level. Most electronics are designed to operate to at least 1000 m without de-rating, therefore you only need to consider the difference from the reference altitude to your actual altitude. So, a piece of equipment with 1000 m service ceiling is only 2000 m above that.

It looks like you used density for you calculation to get 26% difference form 0 to 3000 m, but at 1000 m the difference in density is already over 9%. So, the difference from the normal operating condition (1000 m or less) is only 17%.

Also, not all heat is lost as a result of convection, some is lost through radiation and conduction. As I mentioned before lower ambient temperatures (typically 2C for each 1000 m) can also be used to your advantage.